Ellison broke into print back in 1956, and in the ten years that followed, he wrote an extraordinary number of short stories. Yet prolific as he was from the beginning, he didn't start to hit the level of quality we associate with his mature work until the late '50s. Even then, a number of his stories in speculative fiction would contain scenes, images, and ideas that no one else could handle in those days, only to be marred somewhat by immaturity. This shows up in a lot of the pulp-sf appurtenances that he loaded the earlier stories with in this book. Stories like "The Discarded" and to a lesser extent, "Deeper Than the Darkness" touch places in the psyche that evoke genuine pain--then jar us with risible notions of mutants and somewhat puerile descriptions intended to horrify and shock.
Later stories in this book, such as "Bright Eyes" (1964) and "Repent, Harlequin!..." (1965) are more accomplished and controlled in this regard. I can still read them with considerable pleasure. "Deeper Than the Darkness" has a lot that can be said for it, too, but one must look past some of the pulp-sf crudities that I alluded to above.
The title story, "Paingod," is an interesting attempt that I don't think comes off entirely. A pretty good read, nevertheless.
So you get the picture: the book is a mixed bag, but an interesting one. The stories are often moving, because Ellison felt strongly about the issues embodied, and communicated it effectively. And the book is historically interesting, because it points the way to his later fiction.
Is the book up to the level of quality to be found in his later books, such as _Shatterday_ or _Angry Candy_? No, but that doesn't make it bad. If memory serves, Ellison was 31 when _Paingod_ appeared. The late Theodore Sturgeon wrote an appreciative review at the time where he recognized Ellison's promise on the basis of this book. Neither Sturgeon's probity nor his perspicacity are in doubt because of it. :-)
All in all, the book is worth reading if you're an aficionado of speculative fiction. Ten years ago, I might have said that some of these stories had become dated in a bad way. The cycle of world events seems to have swung back around, and the stories seem (to use a very '60s word) _relevant_ again. :-]